Detection Bias and Overestimation of Bladder Cancer Risk in Type 2 Diabetes: A matched cohort study

OBJECTIVE: To investigate whether the risk of bladder cancer in individuals with newly diagnosed type 2 diabetes is influenced by the frequency of physician visits before diagnosis as a measure of detection bias. RESEARCH DESIGN AND METHODS: With the use of linked administrative databases from 1996 to 2006, we established a cohort of 185,100 adults from British Columbia, Canada, with incident type 2 diabetes matched one to one with nondiabetic individuals on age, sex, and index date. Incidence rates and adjusted hazard ratios (aHRs) for bladder cancer were calculated during annual time windows following the index date. Analyses were stratified by number of physician visits in the 2 years before diabetes diagnosis and adjusted for age, sex, year of cohort entry, and socioeconomic status. RESULTS: The study population was 54% men and had an average age of 60.7 ± 13.5 years; 1,171 new bladder cancers were diagnosed over a median follow-up of 4 years. In the first year after diabetes diagnosis, bladder cancer incidence in the diabetic cohort was 85.3 (95% CI 72.0–100.4) per 100,000 person-years and 66.1 (54.5–79.4) in the control cohort (aHR 1.30 [1.02–1.67], P = 0.03). This first-year increased bladder cancer risk was limited to those with the fewest physician visits 2 years before the index date (≤12 visits, aHR 2.14 [1.29–3.55], P = 0.003). After the first year, type 2 diabetes was not associated with bladder cancer. CONCLUSIONS: The results suggest that early detection bias may account for an overestimation in previously reported increased risks of bladder cancer associated with type 2 diabetes.